Issue 22834. Disable 'Add type annotation' Quick Assist if the type is not accessible.

pkg/analysis_server/lib/src/services/correction/assist_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/assist_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
index 9f5bb8790815fb5bc3d600e99e19747f7c0bc94c..3bed9ab989b22baad6804a8d3253d43cdd1c7209 100644
--- a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
@@ -204,6 +204,11 @@ class AssistProcessor {
       _coverageMarker();
       return;
     }
+    // type source might be null, if the type is private
+    if (typeSource == null) {
+      _coverageMarker();
+      return;
+    }
     // add edit
     Token keyword = declaredIdentifier.keyword;
     if (keyword is KeywordToken && keyword.keyword == Keyword.VAR) {
@@ -248,6 +253,11 @@ class AssistProcessor {
       typeSource = utils.getTypeSource(type, librariesToImport);
       addLibraryImports(change, unitLibraryElement, librariesToImport);
     }
+    // type source might be null, if the type is private
+    if (typeSource == null) {
+      _coverageMarker();
+      return;
+    }
     // add edit
     _addInsertEdit(name.offset, '$typeSource ');
     // add proposal
@@ -297,6 +307,11 @@ class AssistProcessor {
       _coverageMarker();
       return;
     }
+    // type source might be null, if the type is private
+    if (typeSource == null) {
+      _coverageMarker();
+      return;
+    }
     // add edit
     Token keyword = declarationList.keyword;
     if (keyword is KeywordToken && keyword.keyword == Keyword.VAR) {